The present application relates to digital data transmission systems and, more particularly, to a novel clear channel detector for use in determining that a common transmission channel is presently clear, whereby collisions between a plurality of simultaneously-transmitting data stations may be avoided.
In digital data communications systems having a plurality of transceivers connected to a common transmission medium, it is known to resolve collisions, occurring when a plurality of stations simultaneously attempt data transmission, by a carrier-sensing process. One carrier-sensing method has been to monitor an automatic gain control signal in the data receiver, to determine the strength of the received signal. A threshold is set such that the automatic gain control voltage will be above the threshold when a station in the system is actually transmitting and will be below the threshold when all stations are quiet and only noise is being received. Therefore, the decision as to whether or not another station in the system is actually transmitting is based upon the absolute amount of signal received at a particular station. If the transmission medium attenuation varies widely between stations, a transceiver using automatic gain control monitoring and a fixed threshold will not perform in the desired manner, because a weak signal will always be classified as noise, while a strong signal will be classified as a message even if that strong signal is composed entirely of noise. This is especially true in a power line carrier communications system which uses the power wiring, in a building and the like, as the communications medium; impedance modulation caused by other devices on the power circuits can cause rapid fading, of greater than 20 dB., during a single data bit-time interval.
For example, in the bit-echoed arbitration digital data transmission system described and claimed in co-pending application Ser. No. 322,495, filed Nov. 18, 1981, assigned to the assignee of the present invention and incorporated herein by reference, one of the transceivers in a multipletransceiver digital data transmission system acts as an echoer to resolve collisions when a plurality of transceivers simultaneously initiate data transmission on the common system media. During each bit-time interval, the echoer transceiver makes a decision as to which binary symbol, a "1" symbol or a "0" symbol, was received and retransmits, or echoes, the received bit symbol according to that decision. Since the decision is binary, either a "1" or "0" signal is echoed even if there is no transceiver then transmitting and the signal actually received at the echoing transceiver has only noise content. It would be advantageous for the echoing transceiver to be dormant when all of the stations in the system are inactive. Reliable recognition that at least one other station is transmitting a binary data symbol will permit the echoer to be active during transmission and to be dormant when the system is not in active use. Accordingly, it is desirable to provide a detector for recognizing when the common communication medium channel is clear of use.